Carbon nanotubes are unique carbon-based, molecular structures that exhibit interesting and useful electrical properties. There are two general types of carbon nanotubes, referred to as multi-walled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs). SWNTs have a cylindrical sheet-like, one-atom-thick shell of hexagonally-arranged carbon atoms, and MWNTs are typically composed of multiple coaxial cylinders of ever-increasing diameter about a common axis. Thus, SWNTs can be considered to be the structure underlying MWNTs and also carbon nanotube ropes, which are uniquely-arranged arrays of SWNTs.
SWNTs exhibit interesting and useful electrical properties, and may be utilized for a variety of devices, such as integrated molecular electronic devices and others. These devices are applicable to a variety of implementations, such as for sensing chemical and biological species in military missions, defense and protection, environmental monitoring, medical/clinical diagnosis and biotechnology for gene mapping and drug discovery. For instance, detecting chemical weapons in warfare and terrorist attacks are critical to self-alarming and protection. During the Desert Storm conflict, many US soldier were exposed to toxic chemical agents and suffered the Gulf War Syndrome. Other examples include the Tokyo subway poisoning with sarin and the anthrax distribution shortly after the Sep. 11, 2001 terrorist attack in the United States. Advanced chemical sensors are thus highly desired for early and rapid detection and protection in a variety of situations.
Sensors are also important in peaceful environments, such as in medical and biotechnology applications. Useful characteristics of such sensors include the following: (1) fast response, (2) high sensitivity with large response signal of the transducer elements, (3) high selectivity so that the sensor can recognize a specific chemical species, (4) capability of detecting and recognizing as many chemicals as possible, (5) low temperature operation and (6) small, lightweight, compact and convenient to use. Achieving these characteristics, however, has been challenging in previous sensor applications. For example, conventional electrical sensors typically operate at temperatures over 400° C., with a resistance response that is not necessarily significant enough to achieve desired molecular detection. These and other factors have presented challenges for chemical and biological sensing applications.